Dose distributions associated with advanced radiation treatments (e.g. intensity modulated radiation therapy, IMRT) typically exhibit complicated steep dose gradients that conform to irregular anatomical surfaces in three-dimensions (3D). Comprehensive verification is difficult to achieve with conventional dosimeters presenting an immediate and substantial problem. In short, rapid advances in the technology to deliver radiation treatments have not been paralleled by corresponding advances in the ability to verify these treatments. A potential solution has emerged in the form of 3D gel-dosimetry utilizing optical-computed-tomography (optical-CT). It is the long-term objective of this proposal to investigate, optimize and develop gel-dosimetry to a level where accuracy is comparable to that of other standard relative clinical dosimeters (e.g. ion-chambers at 3%) while maintaining the unique feature of high spatial resolution in 3D (Imm3 or better). Should this be possible, it would represent a key goal for radiation dosimetry, and could significantly improve and influence clinical practice. The proposal includes the development and use of a model to study the fundamentals of light transport through the gel and scanning system and the transfer of signal and noise to dose reconstruction. The model will also enable optimization of the system and provide a platform of knowledge to guide in the refinement of future gel dosimeters. Gel-dosimeters (polymer and radiochromic gels) will be characterized according to the physical factors affecting accuracy. Finally, an optical-CT gel-dosimetry system is constructed that is capable of high-resolution 3D dose measurement with accuracy consistent with other standard dosimeters. The utility, accuracy and feasibility of the finalized system will be demonstrated by application to dosimetric verification of advanced IMRT, radiosurgery and brachytherapy treatments in the clinic. Completion will result in a 3D dosimetry system capable of unprecedented comprehensive dosimetric verification, and applicability to the spectrum of modern radiation treatments, including IMRT, brachytherapy, radiosurgery and orthovoltage teletherapy.